1. Field of the Invention
The present invention relates to a positioning system of wireless terminal, and more particularly, a positioning system of a wireless terminal using a satellite positioning system such as the Global Positioning System. The present invention may be directed to the positioning system, a position data server in the system, a wireless terminal in the system, a computer program products for the position data server, and a computer program products for the wireless terminal.
2. Related Art
Terminals having wireless communication functions are called wireless terminals. Mobile phones, mobile terminals and portable computers are among the known wireless terminals. Also known are positioning systems for pointing the location of terminals. For example, there are known positioning systems which use satellites. Specifically, the Global Positioning System is available for use.
These positioning system operates to point the location of a mobile phone based on the satellite signals from GPS satellites when the mobile phone is in such a place as outdoors where it can search for GPS satellites certainly, or based on the signal delay values of the station signals from surrounding stations when the mobile phone is in such a place as the inside of building where it cannot search for GPS satellites certainly.
A mobile phone, if it is designed to point its location by itself, would need to keep almanac data of GPS satellites and almanac data of stations and implement the intricate positioning computation, resulting adversely in a higher cost and higher complexity of the mobile phone. In coping with this situation, there has been devised a system in which each mobile phone does not points its location by itself, but it searches for GPS satellites and stations thereby to fetch GPS satellite data and station data and sends the fetched GPS satellite data and station data to a position data server, and the position data server points the location of the mobile phone based on the GPS satellite data and station data received from the mobile phone.
In this system, in which the position data server points the location of a mobile phone, the mobile phone designates a surrounding station with the smallest signal delay value, for example, to be the reference station from among several surrounding stations near the station in communication linkage with the mobile phone, and sends station data pertinent to the surrounding stations to the position data server.
The position data server receives the station data from the mobile phone, makes reference to the stored almanac data of stations thereby to extract the latitudes and longitudes of the surrounding stations based on the received station data, and computes the pseudo distances from the mobile phone to the surrounding stations based on the signal delay values of the station signals from the surrounding stations relative to the signal delay value of the station signal from the reference station thereby to determine the approximate location of the mobile phone. The position data server sets a search timing and search time which are optimal for the mobile phone to search for GPS satellites, and sends assistant data inclusive of the set-up search timing and search time to the mobile phone.
The mobile phone receives the assistant data from the position data server, extracts the search timing and search time from the received data, searches for GPS satellites based on the extracted search timing and search time thereby to fetch GPS satellite data, and sends the fetched GPS satellite data to the position data server. The position data server receives the GPS satellite data from the mobile phone, points the location of the mobile phone based on the received GPS satellite data, and sends a positioning result indicative of the pointed location of mobile phone to the mobile phone.
At the determination of the approximate location of the mobile phone by the position data server, there arises a position inference error TP which is the difference between the approximate location and the actual location of the mobile phone. The position inference error is large when the number of station data received by the position data server from the mobile phone is small, i.e., station data is below a prescribed level (inadequate). At the transaction of station data and assistant data between the mobile phone and the position data server via the communication station, there arises a propagation delay error TD which is the difference between the timing of transmission of the station signal by the communication station and the timing of reception of the station signal by the mobile phone. The propagation delay error is large when the distance between the mobile phone and the communication station is large.
FIG. 10, FIG. 11 and FIG. 12 show the relation among the position inference error TP, propagation delay error TD, and the search timing TM and search time TS set by the position data server. In these figures, shown by (a) is the timing TT of transmission of the station signal by the communication station, (b) is the timing TR of reception of the station signal by the mobile phone, (c) is the ideal timing TB of search of GPS satellites, (d) is the search timing TM set up by the position data server, (e) is the search time TS set up by the position data server, and (f) is the actual search time TS of the mobile phone and the search timing TMD which is a center value of the search time TS. The figures show a relatively small propagation delay error TDS and a relatively large propagation delay error TDL, and show a relatively small position inference error TPS and a relatively large position inference error TPL.
The position data server sets a GPS satellite search time TS for the mobile phone, which is derived from a certain time range around the center of search timing TM determined based on the inclination angle of GPS satellites and the like. As shown in FIG. 10, in case the propagation delay error TDS and position inference error TPS are relatively small, the ideal GPS satellite search timing TB is within the search time TS of the mobile phone. Accordingly, it is possible for the mobile phone to search for GPS satellites at the ideal GPS satellite search timing TB.
However, as shown in FIG. 11, if the number of station data informed by the mobile phone to the position data server is small, i.e., station data is below a prescribed level, the position inference error TPL increases, causing the ideal search timing TB to be out of the search time TS set by the position data server, and the mobile phone will fail in the search of GPS satellites and thus in the accurate location pointing.
In case the distance between the mobile phone and the communication station is large, the propagation delay error TDL increases, causing also in this case the ideal search timing TB to be out of the search time TS set by the position data server as shown in FIG. 12, and the mobile phone will fail in the search of GPS satellites and thus in the accurate location pointing. The mobile phone will obviously encounter the same situation when both of the position inference error TP and propagation delay error TD increase.